This invention relates to the administration of a therapeutic gas such as nitric oxide (NO) to patients for therapeutic effect. In particular, it relates to a delivery system wherein a controlled pulse of NO is provided to the patient with each inhalation by the patient and to the use of various functions utilized by that system to control and/or eliminate nitrogen dioxide (NO.sub.2) from the system for safety reasons.
The function of the administration of NO has been fairly widely published and typical articles appeared in The Lancet, Vol. 340, October 1992 at pages 818-820 entitled "Inhaled Nitric Oxide in Persistent Pulmonary Hypertension of the Newborn" and "Low-dose Inhalational Nitric Oxide in Persistent Pulmonary Hypertension of the Newborn" and in Anesthesiology, Vol. 78, pgs. 413-416 (1993), entitled "Inhaled NO--the past, the present and the future".
The actual administration of NO is generally carried out by its introduction into the patient as a gas and commercially available supplies are provided in cylinders under pressure and may be at pressures of about 2000 psi and consist of a predetermined mixture of NO in a carrier gas such as nitrogen. A pressure regulator is therefore used to reduce the pressure of the supply cylinder to working levels for introduction to a patient.
The concentration administered to a patient will vary according to the patient and the need for the therapy but will generally include concentrations at or lower than 150 ppm. There is, of course, a need for that concentration to be precisely metered to the patient since an excess of NO can be harmful to the patient.
One current known method and apparatus for the administration of NO to patients is described in U.S. Pat. No. 5,558,083 where a system is provided that can be added to any ventilator and which will meter in the desired concentration of NO into the gas supplied from that ventilator.
Various other delivery devices have also been used that respond to the patient attempting to inhale to deliver a pulsed dose of NO to the patient and such pulsing devices have also been shown to have therapeutic effect on the patient, for example, as described in Higenbottam PCT patent application WO 95/10315 and the publication of Channick et al "Pulsed delivery of inhaled nitric oxide to patients with primary pulmonary hypertension", Chest/109/June 1996. In such pulsatile dosing devices, a pulse of NO is administered to the patient as the patient inhales spontaneously.
The inhalation pulsing type devices are typically shown and described in Durkan, U.S. Pat. No. 4,462,398. Another such apparatus is described in copending U.S. Patent application entitled "Constant Volume NO Pulse Delivery Device", U.S. Ser. No. 08/857,924 and owned by the same assignee as the present application.
One difficulty with such devices that provide a supplemental therapeutic gas to the patient concerns the formation of NO.sub.2 from NO. NO.sub.2 is a toxic compound and its presence is, therefore, undesirable in any appreciable concentration in the gas administered to the patient. Such toxic effects are present at concentrations of about 3 ppm and therefore even minute quantities of NO.sub.2 cannot be tolerated.
In the pulse dose devices that administer NO as a supplemental therapeutic gas to the patient, there is likely to be no monitor to sense the presence of NO.sub.2 and therefore it is important to take preventative measures in the system itself to assure that the formation of NO.sub.2 does not occur, or when it does occur, to remove the NO.sub.2 from the system before the NO containing therapy gas is delivered to the patient.
The formation of NO.sub.2 results from the reaction of NO with O.sub.2 and therefore there is ample opportunity in the administration of NO to a patient for NO.sub.2 to be formed since, obviously, any such administration to a patient must be accompanied by a supply of oxygen to that patient. In addition, the reaction of NO and O.sub.2 to form NO.sub.2 is a time related reaction, that is, the more the NO is in association with the O.sub.2, the more NO.sub.2 is formed, therefore it is important to provide prevention measures wherever there is any time period where the NO and O.sub.2 can be in contact with each other.
Therefore, one of the critical times that the formation of NO.sub.2 can take place and rise to a potentially toxic concentration is in the administration of pulses of NO containing gas where the pulse is triggered by the breath of the patient and where too long a period of time elapses between pulses, that is, for some reason the patient has not triggered a pulse to deliver the NO containing gas to that patient. Since the reaction of NO and O.sub.2 is well known and the volume of the NO device relatively easy to determine, it can be calculated for such devices the critical amount of time that can elapse between pulses of NO containing therapeutic gas before the potential of toxic levels of NO.sub.2 can occur. Accordingly by knowing that time, it is prudent to take preventative measures to insure that the residence time of the NO and O.sub.2 containing components do not remain in any of the passageways of the NO pulse administering device for that period of time.